Zinc binding (ZB) domains are found in numerous proteins which are involved in protein-nucleic acid or protein-protein interactions. ZB proteins are commonly involved in the regulation of gene expression, and may serve as transcription factors and signal transduction molecules. A ZB domain is generally composed of 25 to 30 amino acid residues which form one or more tetrahedral ion binding sites. The binding sites contain four ligands consisting of the sidechains of cysteine, histidine and occasionally aspartate or glutamate. The binding of zinc allows the relatively short stretches of polypeptide to fold into defined structural units which are well-suited to participate in macromolecular interactions (Berg, J. M. et al. (1996) Science 271:1081-1085).
Classes of ZB domains are characterized according to the number and positions of the residues involved in the zinc atom coordination. ZB domains of the C.sub.2 H.sub.2 type were first identified in the protein transcription factor IIIA (TFIIIA; Hanas, J. et al. (1983) J. Biol. Chem. 258:14120-14125) and represent the most abundant DNA binding motif in eukaryotic transcription factors (Berg, supra). These domains, also known as "zinc fingers", are characterized by tandem arrays of sequences that approximate the consensus sequence (Tyr, Phe)-X-Cys-X.sub.(2-4) -Cys-X.sub.3 -Phe-X.sub.5 -Leu-X.sub.2 -His-X.sub.(3-5) -His, wherein X represents a more variable amino acid. The cysteine and histidine residues coordinate a zinc ion, the three other conserved residues form a hydrophobic core adjacent to the metal coordination unit, and the variable amino acids mediate interactions with other molecules. The overall structure consists of two antiparallel .beta.-strands adjacent to an .alpha.-helix (Berg, supra). A protein may contain one or more zinc fingers which interact independently of each other. In many instances, proteins which contain zinc finger domains interact with specific double-stranded DNA (dsDNA) sequences, and carry out roles as transcription factors. Some zinc finger proteins, such as TFIIIA, bind to both dsDNA and to single-stranded RNA, while others, such as p43, appear to bind only to single-stranded 5S RNA (Berg, supra). Furthermore, certain zinc finger proteins, including the human transcription factor SP1, bind DNA-RNA heteroduplexes with affinities comparable to or greater than those for DNA duplexes (Shi, Y. et al. (1995) Science 268:282-284).
A variant of the zinc finger described by a C.sub.2 C.sub.2 sequence motif is found in the Xenopus G10 protein (McGrew, L. L. et al. (1989) Genes Dev. 3: 803-815). G10 mRNA is a maternal transcript that is translationally activated during oocyte maturation. G10 protein consists of N-terminal containing a nuclear translocation signal (NTS) and alternating acidic and basic residues, and C-terminal sequence containing the C.sub.2 C.sub.2 -type zinc finger motif. G10 appears to function as a nuclear regulatory protein (McGrew et al., supra). Sequences highly homologous to G10 have been found in various organisms, including C. elegans, rice, and S. cerevisiae (Benit, P. et al. (1992) Yeast 8:147-153).
ZB domains which contain a C.sub.3 HC.sub.4 sequence motif are known as RING domains (Lovering, R. et al. (1993) Proc. Natl. Acad. Sci. USA 90:2112-2116). The RING domain binds two zinc ions in an arrangement structurally different from that of the zinc finger. The RING domain consists of eight metal binding residues, and the sequences that bind the two metal ions overlap (Barlow, P. N. et al. (1994) J. Mol. Biol. 237:201-211). The consensus sequence C-X.sub.2 -C-X.sub.(9-27) -C-X.sub.(1-3) -H-X.sub.(2-3) -C-X.sub.2 -C-X.sub.(4-48) -C-X.sub.2 -C provides for loops of varying length which form the overlapping Zn binding sites. The two Zn binding sites are formed by four pairs of metal-binding Cys and His residues. The first and third pairs bind one metal ion, while the second and fourth pairs bind the other (Barlow, et al., supra). Functions of RING finger proteins are mediated through DNA binding and include the regulation of gene expression, DNA recombination, and DNA repair.
The murine BMI-1 gene encodes a protein of 324 amino acids. This protein, which is found in the nuclei of a variety of normal cells, contains a RING domain near the aminoterminus (Haupt, Y. et al. (1991) Cell 65:753-763). Retroviral insertional mutagenesis of E-mu/myc transgenic mice by infection with Moloney murine leukemia virus (MuLV) accelerates development of B lymphoid tumors. In about half of independently induced pre-B-cell lymphomas, the provirus integrates in or near the BMI-1 gene, which results in enhanced transcription of that gene. Haupt et al. (supra) concluded that myc-induced lymphomagenesis may entail the concerted action of several genes, including the putative nuclear regulator BMI-1. The human BMI-1 gene encodes a protein of 326 amino acids which shares 98% identity to the amino acid sequence of the mouse protein (Alkema, M. J. et al. (1993) Hum. Mol. Genet. 2:1597-1603). Fluorescence in situ hybridization (FISH) on metaphase chromosome spreads localized the human BMI-1 proto-oncogene to the short arm of chromosome 10 (10p13), a region known to be involved in translocations in various leukemias (Alkema et al., supra).
The breast and ovarian cancer susceptibility-1 (BRCA1) gene encodes a predicted protein of 1,863 amino acids which contains a RING domain in the amino-terminal region (Miki, Y. et al. (1994) Science 266:66-71). BRCA1 is expressed in numerous tissues, including breast and ovary. In sporadic breast cancer, BRCA1 mRNA levels are markedly decreased during the transition from carcinoma in situ to invasive cancer (Thompson M. E. et al. (1995) Nature Genet. 9:444-450). Furthermore, experimental inhibition of BRCA1 expression with antisense oligonucleotides produced accelerated growth of normal and malignant mammary cells, but had no effect on nonmammary epithelial cells. Thompson et al. interpreted these results as an indication that BRCA1 may normally serve as a negative regulator of mammary epithelial cell growth and that this function is compromised in breast cancer either by direct mutation or by alterations in gene expression.
A variation of the RING finger motif in which a His replaces the fourth Cys of the consensus (C.sub.3 HHC.sub.3) is found in the protein product of the Drosophila developmental gene goliath (G1; Bouchard M. L. et al. (1993) Gene 125:205-209). The G1 gene is an abundant transcript of the visceral mesoderm of the Drosophila embryo. Mesoderm is one of the fundamental embryonic germ layers which gives rise to internal structures such as the body and gut musculature, fat body and heart. A high frequency of hydrophobic and uncharged residues, primarily Ser, Gln and Pro (SQP-rich region), is found in the last one-third of the G1 protein. Based on the observation that similar domains impart transcriptional activation ability to eukaryotic DNA-binding proteins (Mitchell, P. J. et al. (1989) Science 245:371-378), Bouchard et al. suggest that the SQP-rich region of G1 is a potential transcriptional activation domain.
The discovery of polynucleotides encoding human zinc binding proteins, and the molecules themselves, provides a means to investigate physiological processes relating to the control of cellular differentiation and proliferation under normal and disease conditions. Discovery of novel zinc binding proteins satisfies a need in the art by providing new diagnostic or therapeutic compositions useful in diagnosing and treating diseases relating to disregulated cell growth and proliferation including cancer.